Piezoresistive micromechanical system (MEM) pressure sensors are configured to translate a pressure into an electrical signal corresponding to the pressure. Normally, a piezoresistive MEMs pressure sensor comprises a pliable diaphragm having one or more resistive elements diffused into it. The diaphragm is configured to measure a pressure within an adjacent cavity based upon resistive changes caused by a force that the pressure exerts on the diaphragm. For example, a high pressure within an adjacent cavity causes the diaphragm to bend in a first direction that compresses the resistive element, thereby decreasing the resistance. Alternatively, a low pressure in the cavity causes the diaphragm to bend in a second direction that expands the resistive elements, thereby increasing the resistance.
The diaphragm is formed within a silicon substrate using an electrochemically controlled etching (ECE) process. The ECE process utilizes a wafer comprising a p-type substrate with an n-type epitaxial layer formed thereupon. During etching, the wafer is immersed in an etchant (e.g., a KOH bath), and a controlled bias is applied between the n-type epitaxial layer and the etchant. A PN junction, formed at the border of the n-type epitaxial layer and the p-type substrate, acts as a reverse-biased diode preventing the etchant from being exposed to any current, so that the etchant performs a normal anisotropic etch through the substrate until it reaches the junction. Then, because the diode has been etched away, the etchant is exposed to the applied bias, and a reaction forms a thin layer of silicon dioxide on the substrate, which stops the etch.